Fixing a Leaking Stopcock: A Simple Guide

Fixing a leaking stopcock doesn’t always require calling in a plumber. 

You might be able to do it yourself if you can identify the right stopcock parts and use some basic tools.

Stopcock: A definition

A stopcock (sometimes called a stop tap) is the mains valve that controls the flow of water into your property.

It is always switched “on”. But you can turn it off for several reasons including if you:

• Have a water leak elsewhere in your property (i.e., not from the stopcock itself)
• Are carrying out repairs (i.e., removing airlocks in pipes)
• Doing maintenance or home improvements.

A functioning stopcock is the quickest way to turn off the water supply to your home if you have a plumbing emergency (like a burst pipe or broken internal tap).

Moreover, a faulty stopcock can affect water pressure throughout your home.

So, for example, if your toilet won't flush properly after a stopcock issue, low mains pressure may be the cause

Did you know? Throughout history, it was the norm for many inventions with no known inventors to appear. The stopcock is one of them. Its earliest known form was the Roman plug cock used to control water flow to public buildings and homes as early as the 1st century AD. Its basic design remained largely unchanged right up until the 19th century.

Where to find your stopcock

There are two stopcocks for your home. The internal one is the one most homeowners are concerned with.

Internal stopcocks

In most properties, the easiest place to find this is under the kitchen sink, usually in a cupboard or near where the water pipe enters your home.

If it is not there, check these other common areas:

• Downstairs toilet or utility room
• Under-stairs or airing cupboard.
• Basement or under floorboards near your front door.
• Garage.

In older houses, stopcocks are sometimes boxed in during renovations. If you still cannot find it, ask your neighbours, as their pipe layout is likely similar. 

And in new builds they are often sleeker and hidden behind access hatches rather than being instantly visible. 

External stopcock

This is the responsibility of your water supplier to fix if there is a leak, not you. Remember, this impacts other homes in your area, too. 

It's located outside the property, near your boundary, usually in a meter pit on your driveway, grass verge, or pavement.

It will have a metal or plastic cover marked with 'W' or 'Water'.

Did you know? True stopcocks (the plug-style valve) don't scale up in size much. Friction sealing between the plug and body limits them to roughly 100mm diameter and modest pressures and household versions are usually just 15-28mm. Beyond this size, butterfly valves are used. They have the same principle of using rotation to block water flow, but use a different mechanism. The largest ever built in the US was a butterfly valve installed at a water treatment plant in 2021. It stands 17 feet tall and weighs around 70 tons.

Compression nuts

If water is coming out of your stopcock, then the problem is either with the compression nuts or the gland nut (see below). 

There are two compression nuts on either side of the stopcock which join the tap to the water pipes on either side.

Fixing compression nuts

Step 1: The basic fix

Fixing compression nuts is easy and usually only requires water pump pliers and a spanner. If you can see water leaking from the compression nuts on either side of the stopcock, you may just need to tighten them. 

To do this, grip the body of the stopcock with the pliers and use your spanner to turn the compression nut in a clockwise direction.

Step 2: Creating a seal

If step 1 doesn’t stop the leak, you may need to create a seal with PTFE tape.

1. Switch off the water supply to the stopcock. Do this by turning off the external stopcock. This may be located in a drain outside your home.
2. Drain the water in the system. Turn on a tap and wait for the system to empty.
3. Loosen the compression nut. Grip the stopcock with the pliers and use the spanner to loosen the nut.
4. Wrap and replace. Wrap the thread with PTFE tape and then replace the nut.
5. Turn your external stopcock back on.

Gland nut

If water is coming out of your stopcock, and the issue isn’t the compression nuts, it could be the gland nut. 

The gland nut sits below the stopcock. It prevents water from flowing up inside the spindle.

Fixing gland nuts

Step 1: Turn off the external stopcock

Locate and turn off your external stopcock to avoid water leaking during the following steps.

Step 2: The basic fix 

Fixing a gland nut is relatively easy and only requires a spanner. 

Like the compression nut, it can easily be tightened. 

Tighten the gland nut with a spanner. 

Be careful not to tighten this too much as it could prevent you from turning the stopcock.

Step 3: Replace the packing

If step 2 doesn’t fix the problem, you may need to replace the packing. 

This is a more difficult task that requires spanners, a screwdriver, and PTFE tape/packing.

1. Use one spanner to hold the large nut on the stopcock and the second spanner to loosen the gland nut
2. Slide the gland nut up the spindle using a screwdriver to manoeuvre it
3. Remove any old packing debris and then wrap the gland in PTFE tape or packing
4. Gently push this back into the gland and then tighten the gland nut
5. Turn your external stopcock back on.

Head gear joint

If water isn’t coming out but the stopcock doesn’t stop the flow of water when it’s in the “off” position, the problem is likely the head gear joint.

The head gear joint is the mechanism inside the stopcock which stops the flow of water. 

A leaking gear head joint is the result of a broken or eroded washer. 

It might also cause a dripping tap. Whilst you have your tools out, you can potentially fix this, too.)

The washer is located at the base of the spindle and it is the mechanism that stops the flow of water. 

Fixing this is difficult because there are more steps and a higher risk of pipework damage. It also requires more tools:

• Water pump pliers
• Screwdriver
• Spanner
• Replacement washer
• PTFE tape.

The fix: replacing a head gear joint

To replace this, you will need to take the following steps:

1. Turn your external stopcock off to isolate the water to your home and drain the pipes.
2. Loosen the large nut in the centre of the stopcock. Use your spanner and water pump pliers
3. Remove the entire stopcock and spindle. Unscrew the tap to do this
4. Replace the old washer with a new one
5. Wrap PTFE tape around the bottom of the spindle. This is just above the washer. 
6. Replace the spindle
7. Retighten the nut.
8. Turn the external stopcock back on.

How to un-jam a seized stopcock

If your stopcock is seized, you can often free it up yourself with a bit of patience.

First, spray the spindle and packing gland with plenty of WD-40. Let it soak for 10 minutes. 

 Next, use a spanner to loosen the gland nut (see diagram above) by about half a turn. This release pressure on the internal thread. 

After this, tap the valve handle very gently with a spanner or hammer in both directions. This helps break up limescale inside. 

Then, use grips or footprints to turn the handle back and forth. Don’t force it all or you might snap the handle. Instead, work it gradually in small twists, you should be able to feel it getting looser before it turns easily.

Finally, tighten the gland nut back up to stop any dripping and turn the valve fully open, then back it off a quarter-turn.

When repair isn't enough: replacing a stopcock

Replacing the headgear (internal components) is a manageable DIY job for people comfortable with basic plumbing. However, replacing the full valve body is highly difficult and risky, so calling a plumber for this is the sensible choice.

To replace a jammed or leaking stopcock,  the mains water supply at the external boundary valve must first be disconnected.

The most straightforward method is to replace the stopcock’s moving internal components rather than the entire valve (which includes the outer brass shell). 

This involves unscrewing the headgear with a deep socket and winding in compatible new internals. This way avoids cutting into old pipework.

If the whole valve needs to go, both nuts on either side of the stopcock need to be disconnected. 

For old imperial pipework (that is easily scratched), you should carefully cut away the tightly clamped brass rings (known as olives) with a hacksaw and then split them with a screwdriver.

After adding potable-water-approved jointing paste to the new olives to ensure a watertight seal, hold the valve firmly with grips while tightening the nuts.

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